The mouse hemopoietic-specific early response protein, A1, has been studied by Lin et al (1993; J Immunol 151:1979-88) who first reported on its relationship to Bcl-2 (B-cell leukemia/lymphoma 2)and Mcl-1. A1 belongs to a gene family that has been characterized in humans, mice and chickens (Craig RW (1995) Semin Cancer Biol 6:35-43) and includes Bfl-1, Bcl-2, Bcl-x, Bcl-xL, bax, and mcl-1. These genes regulate cell viability; in some cases, preventing toxicity to antibiotics and anticancer drugs (Minn A J et al (1995) Blood 86:1903-1910), and in others, governing the apoptosis necessary for tissue differentiation and organismal development. The exact mechanism by which A1, Bcl-2 and related genes promote cell survival is not known; however, they may function through the inhibition of cysteine proteases.
The coding region of the murine A1 (Lin et al, supra) consists of 648 nucleotides which encode 216 amino acids. The open reading frame (T.sub.474) and the 3' untranslated region (T.sub.715) each contain a TACAAA motif which is found in many immediate early genes and may be essential for induction. The deduced protein has a predicted molecular weight of 20,024, an isoelectric point of 5.05, and a potential glycosylation site at N.sub.128. The absence of a secretion signal led Lin et al (supra) to suggest that A1 might be an intracellular rather than a secreted protein.
Expression of A1 has been detected in bone marrow, spleen and thymus and is induced by GM-CSF (granulocyte-macrophage colony stimulating factor) in several hematopoietic cell lineages, including T-helper lymphocytes, macrophages and neutrophils, and in myeloid cell lines induced to differentiate by IL-3. A1 is induced in a macrophage tumor cell line by lipopolysaccharide. The protein synthesis inhibitor cycloheximide works as an agonist to induce the long-term expression of A1, a feature previously reported for other early response genes (Lin, supra). Neither serum nor the cytokines, interleukin (IL)-1 alpha or IL-6, induce A1 expression.
Calcium dependent or C-type lectin receptors are widely expressed in cells of the immune system. Their characteristic features include: 1) a cytosolic amino terminus containing at least one potential tyrosine phosphorylation site which may be involved in signal transduction and several prolines which may prevent steric interference between the cytosolic and membrane spanning domains, 2) a short, approximately 20, hydrophobic amino acid transmembrane domain, and 3) a series of cysteine residues which appear to function as an extracellular carbohydrate recognition domain (Speiss M (1990) Biochem 29:10009-18). When the extracellular carbohydrate binding domain is separated from the membrane spanning domain by protease activity, it maintains both its structural and functional integrity. As described below, macrophage C-type lectin receptors perform a variety of functions in the recognition and destruction of foreign cells.
Diseases or Activities Associated with A1 or C-lectin Family Genes
Alterations or aberrations in A1 family gene expression are known to result in premature cell death or in cancer. The best known gene in this family, Bcl-2, is a proto-oncogene associated with human follicular lymphoma (Tsujimoto Y and Croce CM (1986) Proc Nat Acad Sci 83:5214), malignant melanomas, and solid tumors such as carcinomas of the lung, prostate and nasopharynx (Cerroni L (1995) Am J Dermatopathol 17:7-11). In patients with AIDS, there is also a high correlation between Epstein-Barr virus (EBV) and primary brain lymphomas. The virus's latent membrane protein 1 has been reported to transactivate the Bcl-2 gene and in one study, both genes were expressed in 10 out of 11 cases of AIDS-related primary brain lymphoma. Expression of Bcl-2 is far less common in systemic lymphomas and cutaneous B or T cell lymphomas (Garatti S A et al (1995) Recent Results Cancer Res 139:249-261).
Estrogen also increases Bcl-2 expression promoting chemotherapeutic drug resistance in an estrogen-responsive human breast cancer cell line (Teizeira C et al (1995) Cancer Res 55: 3902-3907). Bcl-2 is normally expressed in the epithelial regenerative compartment or the basal crypts of the colon and small intestine. Overexpression of Bcl-2 was not seen in inflammatory gastrointestinal conditions such as ulcerative colitis, Crohn's disease, or hamartomatous polyps; but it was common in hyperplastic colonic polyps and in the majority of dysplastic lesions, adenomas and adenocarcinomas. The presence of excess Bcl-2 in tissue surrounding the lesions suggests that the neoplasias arose from tissue in which earlier conversion to abnormal Bcl-2 expression occurred (Bronner M P et al (1995) Am J Pathol 146: 20-26).
Some of the other A1 family genes are now being characterized. Clinical studies on the gene, Bfl-1 (Choi S S et al (1995) Oncogene 11: 1693-98) isolated from human fetal liver and highly expressed in bone marrow have shown a correlation between expression of Bfl-1 and stomach cancer. The studies suggest that Bfl-1 may promote the survival of stomach cancer cells by preventing apoptosis. Expression of Bcl-XL dramatically reduces cytotoxicity to antibiotics and chemotherapeutics such as bleomycin, cysplatin, hygromycin, and vincristine, (Minn, supra; Newcomb E W (1995) Leuk Lymphoma 17: 211-221).
Silvestris F et al (1995; Ann Ital Med Int 10: 7-13) showed that A1 family genes are involved in autoimmune conditions such as lupus erythematosus and degenerative neuropathies such as Alzheimer's disease; and Erlacher et al (1995; J Rheumatol 22: 926-931) reported on the increased expression of Bcl-2 in chondrocytes adjacent to osteoarthritic defects. Other studies suggest that inducing the expression of A1 family genes may serve to rescue neurons from programmed cell death due to hypoxia (Shimizu et al (1995) Nature 374:811-816) or cerebral ischemic stroke (Linnik M D et al (1995; Stroke 26:1670-74).
Unique C type lectin receptors may direct the macrophages to abnormal or diseased cells where they specifically interact with surface antigens. For example, Klebsiella pneumoniae serotypes displaying certain surface mannose polysaccharide sequences bind to and are subsequently internalized and destroyed by macrophages (Athamna A et al (1991) Infect Immun 59: 1673-1682). Similarly, the Tn Ag of a well-known human carcinoma-associated epitope (Suzki N et al (1996) J Immunol 156: 128-135) is recognized by a human macrophage C-type lectin. Binding of macrophages to mastocytoma cells occurs through the Gal/GalNAc-specific macrophage lectin and activates the tumor cell killing mechanism (Oda S et al (1989) J Biochem 105: 1040-1043).
Some diseases are due to defects in the recognition of what is foreign as mediated via the macrophage lectin receptor, and some conditions such as graft and transplant rejection and pathogen colonization of host macrophages derive from normal, yet undesirable, functioning of macrophages. For example, in rat cardiac allografts, expression of macrophage cell-surface lectins were linked to chronic rejection. In this case the lectin served as a possible mediator of macrophage infiltration (Russel M E et al (1994) J Clin Invest 94: 722-730). Also, the attachment of such pathogens as Mycobacteria tuberculosis via mannose-specific lectin receptors expressed on the macrophages (Goswami S et al (1994) FEBS Lett 355: 183-186) is the preliminary step in pathogenesis.
Macrophage antigens/lectins clearly play an important role in the recognition and destruction of foreign and diseased cells. The selective modulation of the expression and specificity of a novel human macrophage antigen may allow the successful management of diseases related to macrophage function, allowing natural cytolysis via specific targeting to infected host cells or tumors or preventing graft rejection and pathogen colonization.